A cephalosporin antibiotic has an excellent antibacterial activity and a low toxicity to mammals, and therefore it is an extremely effective medicine useful for the therapeutic treatment of bacterial infections in the mammals. In recent years, many cephalosporin derivatives having an aminothiazolylacetyl group at the 7-position of the cephem ring have been researched and developed, because they have a strong antibacterial activity and a stability to .beta.-lactamase.
The so-called third-generation cephalosporin antibiotics represented by cefotaxime and cefmenoxime have the aminothiazolylacetyl group at the 7-position and are characterized by a high antibacterial activity and wide antibacterial spectra, and so they have been practically used in many countries of the world. However, some compounds amongst the third-generation cephalosporin antibiotics such as cefotaxime and cefmenoxime are not satisfactory in points of their antibacterial activity to Pseudomonas aeruginosa and methicillin-resistant Staphylococcus aureus which clinically provide some problems in the recent years. In particular, the methicillin-resistant Staphylococcus aureus brings about a serious bacterial infection, and nowadays it is desired to provide a novel cephalosporin antibiotic having an improved antibacterial activity to these resistant bacteria.
Incidentally, one of the present inventors and his colleague have earlier succeeded in synthetizing a new class of cephalosporin derivative of general formula (A): ##STR2##
wherein R.sup.1 is a hydrogen atom or a lower alkyl group, R.sup.2 is a hydrogen atom or an ester-forming group capable of being cleaved easily with an esterase existing in the digestive tracts; n is an integer of zero or 1; Z is a saturated heterocyclic group containing one or two oxygen atoms as the hetero-atoms with or without one or more lower alkyl substituents, or a pharmaceutically acceptable salt thereof (U.S. patent application Ser. No. 623,215 filed Dec. 6, 1990, now U.S. Pat. No. 5,332,731 and European patent application publication No. 0 432 042 A2).